Many techniques are available for controlling the average power from an AC voltage source power supply to an electrical load. One of the most efficient methods is the use of a series deployed synchronously operated two-state (on-off; or, open-closed; or, conducting-non conducting) switching device wherein the average voltage per AC half wave cycle applied to the load, and thus the average real power, is varied by controlling the relative timing at which the switch changes from one of its two states, to the other. These techniques are loosely termed "AC Phase Control" when really they are a form of Pulse Time Modulation (PTM) where the carrier is an impulse wave train marking the zero crossings, positive going and negative going of the sinusoidal voltage wave. The parameter being modulated is usually the duration (sometimes termed width) of one or the other of the two states of the switching device relative to the above impulse wave train to define the Pulse Duration Modulation (PDM)variant of PTM. Then two distinct forms of PDM ("AC Phase Control") are definable. The first can be described as when the conducting or "on" state of the switch follows a timed non-conducting or "off", state of the switch which starts at a time index marked by the impulse wave train. This control methodology is termed Normal or Direct Phase Control and is particularly well suited to regenerative switching devices of the semiconductor world, classed as Thyristor structures (PNPN) which generally cannot be returned to their non-conducting state with a signal at the control port (the "gate electrode") but recover naturally to their non-conducting state each time the controlled current approaches zero. Hence, turn-off occurs near or at the end of every half cycle of conducted current. The second form of AC Phase Control is called Inverse or Reverse AC Phase Control because it has the opposite relationship of conducting and non-conducting periods of the synchronous switch. Consequently this class of control cannot use switching devices of the Thyristor class as they normally, i.e., "easily", cannot be turned off through action on a control port (the "gate" electrode). Therefore, this form of AC Phase Control requires the use of non-regenerative switching devices of the transistor class, either of bipolar and unipolar (FET) junction structures.
A further generalization within the above particular form of PDM construct to a more general form of PDM is to vary the instant of transition between the conducting and non-conducting states of the synchronous switch, nominally identified as turn-on and turn-off with their obvious connections to the states, such that both, not just one, do not necessarily coincide with an AC voltage wave zero crossing.
Thus there are four basic turn-on/turn-off combinations within the PDM control discipline. For purposes of exposition, in the ensuing discussion gamma (.gamma.) will correspond to "turn-on" and delta (.delta.) will correspond to "turn-off". Note that gamma and delta may be interchanged in the course of an AC half wave and further, may be repeated during the half wave period. Furthermore, and as noted above, the two special cases of Normal and Reverse AC Phase Control correspond to one of the instants occurring at the zero crossing.
______________________________________ CHARACTERIZING CASES: .delta. = turn-off, .gamma. = turn-on NOMENCLATURE ______________________________________ 1. .gamma. before .delta. and both variable ON-OFF "NOTCH" 2. .delta. before .gamma. and both variable OFF-ON "NOTCH" 3. .delta. = carrier wave zero cross NORMAL PHASE and .gamma. is variable CONTROL 4. .gamma. = carrier wave zero cross REVERSE PHASE and .delta. is variable CONTROL ______________________________________
The preferred combination of turn-on and turn- off for a given application depends critically upon the desired circuit action and the type of device(s) selected to implement the series disposed synchronous switch. In summary, the above two generic cases, 1. and 2., and the two special cases, 3. and 4., may be usefully termed generalized AC Phase Control with full understanding and recognition that their properties derive from those of Pulse Duration Modulation form of PTM well known from the fundamental theory of Electrical Communication pertaining to Modulation.
In circuit operation involving switching, two basic rules may be surmised, that are readily derivable from the underlying physics of lumped constant electric circuits. The first is: avoid opening a switch in series with an inductor, a true energy storing inductor, while current is flowing. To the extent the switch opening causes di/dt to approach infinity, i.e., the current instantaneously going to zero, a voltage approaching infinity will appear across both the switch as well as the inductor. The second is: avoid closing a switch in shunt with a capacitor which has a stored charge, manifested by a voltage between its terminals. To the extent that the switch closing causes dv/dt to approach infinity, i.e., the voltage instantaneously going to zero, the current flow through the switch approaches infinity. Furthermore, either the stored magnetic, (for an L) or the stored electric energy, (for a C) must perforce be dissipated in the switch. The significance of these two "rules" are explained in more detail in co-pending U.S. patent application Ser. No. 571,830, filed Jan. 19, 1984 and Ser. No. 723,184 filed on Apr. 15, 1985 and now U.S. Pat. No. 4,642,525 issued Feb. 10, 1987 and all the references cited therein and at least recognized degree in U.S. Pat. No. 4,350,935 (Spira et al) and the references cited therein as well as Widmayer U.S. Pat. Nos. 4,352,045 and 4,394,603.
AC power circuits using switching are often implemented with semiconductor devices rated as capable of meeting the steady state operating voltage and current requirements of the application plus some safety margin ultimately limited by economic considerations. However, there are times when current and/or voltage transients, many times the steady state level, are present and, on occasion, they may exceed the economical safety margins provided by a given design. These electrical transients in AC phase control circuits, particularly the generalized ones heretofore discussed, are caused by the generation of unipolar currents, i.e., "DC", transients due to non-synchronous (asynchronous) switching or excessive asymmetric current flow resulting from events relating either to the source of power or the control mechanisms herein considered, or in the electrical load, or, any or all of the combinations thereof. The presence of transients must be anticipated and the circuit must be designed so that the safe operating ratings of its components are not exceeded when the circuit is operating with current or voltage transients that are substantially in excess of the steady state operating levels.
Examples of non-synchronous switching, where current transients may be generated that are in excess of the ability of some semiconductor electronic devices to handle, or, which in turn cause voltage transients, include the turning on or off of the branch circuit by electromechanical switching device to connect a load and its power controller, if any, to the AC voltage source. In this instance unless the branch circuit switching device were to be turned-on exactly at a specified phase angle (related to that of the prior turn-off), it is likely, that a high transformer magnetizing in-rush current transient will occur. This transient problem was identified long ago as due to remnant polarization (i.e., core magnetization) and was cited as a transient inducing problem beginning in Column 14 line 9 of U.S. Pat. No. 4,350,935. Likewise, circuit design considerations and consequent device characteristics often unexpectedly cause circuit consequences such as the large phase reversal current transient which may occur during the first AC cycle in circuits that have a capacitor and a saturable non-linear magnetic core device in series relationship with the potential for unwanted ferroresonance effects to occur. This phase reversal phenomena is explained in further detail in U.S. application Ser. No. 723,184 (filed Apr. 15, 1985) and now U.S. Pat. No. 4,642,525. In addition, a large current transient flowing in a series capacitor/non-linear inductor circuit can develop voltage across the series capacitor which can exceed the ratings of it and any other related circuit components. Still further, and as more fully explained in co-pending application Ser. No. 571,830, filed Jan. 19, 1984, a switch closing around a capacitor with significant stored energy can result in a damaging current transient upon closing of the switch. In addition, the phenomena called ferroresonance, which can create large voltages and ensuing current transients, can cause circuit damage unless protective steps are taken. Further information on this phenomena may be found in co-pending U.S. patent application No. 769,829 filed on Aug. 29, 1985 and now U.S. Pat. No. 4,766,352, issued on Aug. 23, 1988. Non-synchronous switching will also occur during the start-up or shut-down periods of the Power Controller's control signal generation function if proper inhibiting is not included.
A further problem stems from the widespread use of a bipolar transistor as a unidirectional current switching device connected between the DC terminals of a rectifying diode bridge to implement the bi-directional synchronous switch function, particularly in the previously discussed "reverse" or "on"-before-"off" AC phase control circuits. Such a bipolar transistors, used as a unidirectional current switch, needs a base signal capable of driving and maintaining the collector-emitter in saturation in order to minimize power dissipation. However, it is not practical to provide a base drive of sufficient amplitude to keep the transistor in saturation for those occasions where a large current transient has to flow through the collector-emitter (C-E) circuit. Moreover, a large collector current of sufficient duration can result in the opening of the electrode bonding wire element (I.sup.2 t rating) or if large enough to take the transistor out of saturation, can cause melting of the junction structure due to the high instantaneous power being dissipated (exceedance of the safe operating area (SOA) rating).
Nevertheless the bipolar transistor is used in Reverse Phase Control applications because it can be readily turned-off by appropriate electrical drive applied to its base electrode.
Of note, Reverse AC Phase Control circuit operation is characterized by an increasingly leading power factor as the average power flow to the electrical load is reduced, whereas Normal AC Phase Control circuit operation is characterized by an increasingly lagging power distortion factor as the average power flow to the electrical load is reduced. Reverse AC Phase Control circuit operation has been found useful in applications where it is desirable for the control action on the reduced electrical load to provide a leading rather than lagging power factor. This leading characteristic tends to compensate for other lagging loads powered by the same electrical power system. In such Reverse AC Phase Control applications the electronic device implementing the synchronous switch is turned on at the zero crossover of the AC source and turned off at some variable point in time (or phase angle) before the next zero crossover. Since this turn-off control is possible when substantial current can be flowing, care must be taken to prevent destructive voltage transients from developing as a result of the closed circuit having stored magnetic energy properties. This protective function can be a by-product of utilizing a critically valued capacitor that is large enough to cause a beneficial amount of ferroresonance to increase the voltage appearing across the ballast primary winding to aid lamp ignition, to provide a minimum level of RMS voltage to the ballast primary after ignition, to provide an adequate level of lamp cathode heater voltage, where applicable, as well as providing a minimum level of arc current to provide a low level of stable lamp arc operation during periods of the half wave that the synchronous switch is not conducting. Thus the critical value capacitor element associated with the synchronous switch is multipurpose and lamps can be operated without the periods of current discontinuity within the AC half cycle that is normally associated with AC Phase control.
The capacitor must be of a critical value properly related to the parasitic inductive properties of the load circuit. In addition its capacitance value must be sufficiently large so that the voltage developed across the capacitor does not exceed is voltage ratings or the voltage rating(s) of the synchronous switch elements yet not large enough to unnecessarily reduce the control range of the Power Control technique being implemented. With some variation between ballasts models and manufacturers a value around 4 microfarads is useful for an AC load comprised of one 120 volt rapid start ballast and two F40 rapid start fluorescent lamps. This value of capacitance will among others, cause sufficient ferroresonance to occur to provide lamp ignition, continuous arc current during each half cycle to provide stable low level lamp operation, a post ignition regulated voltage on the ballast primary (despite fluctuations of the line voltage) at a level sufficient to provide an adequate heating of lamp cathodes where applicable.
However, the addition of such a critically valued capacitor also necessitates that turn-on of the transistor switch be variable in time to prevent turn-on whenever the capacitor has a substantial stored electric charge, (i.e., a voltage of significance across its terminals) as explained in detail in co-pending U.S. patent application No. 571,830 filed Jan. 19, 1984. Further it can cause damaging consequences to the power controller if the load circuit is energized without the lamp portion of the load. For example, one of the lamps can be removed (which simulates a failed lamp) when the circuit is energized and cause a non-symmetrical voltage buildup across the capacitor can be observed over a period of several AC 1/2 cycles before the buildup exceeds the voltage withstand ratings of the rectifying diode bridge elements or the switching transistor. Since it is also unpredictable as to when a circuit fault might be corrected, e.g., failed or removed lamp or lamps might not be replaced for hours or days, and during such period the ferroresonance effects of voltage magnification and resulting current transients could damage, if not destroy, the control elements if synchronous switching is attempted with the incomplete load circuit. However, the control circuit can be protected from any damaging effects of the ferroresonance by turning the synchronous switch full-on or full-off when such a load fault occurs. When the transistor conducts (the low level magnetizing current) throughout the AC cycle, ferroresonance effects cannot occur, since the capacitor, whose series relationship with the transformer ballast, and its inductive properties is the cause of the ferroresonance, is effectively shorted out of the circuit. Circuits have been demonstrated that turn the synchronous switch, i.e., the switching transistor, full-on when ferroresonance occurs, and generally, are returned to normal operation, by turning power off to unlatch the full-on circuit. Normal operation then occurs at the next power turn-on if the ferroresonance is damped by a proper load circuit being present. Circuits have also been demonstrated where the synchronous switch is turned full-off and while some beneficial ferroresonance is still present damaging current transients are not generated.
In summary, for high reliability of switching circuits employing a by-pass capacitor, operating within an AC half wave, where both control of "on" and "off" is a requirement, a bi-polar transistor, while having the ability to be turned both "on" or " off", has some remaining shortcomings with respect to its ability to be kept within its Safe Operating Area rating(s) whenever current transients are present. It is obvious that a gate controlled thyristor, insulated gate transistor, field effect transistors, or other hybrid semiconductor devices with higher current ratings are candidates for consideration in applications requiring both "ON"-before-"OFF" control. However, these alternative devices usually present other problems such as insufficient voltage withstand and/or high power dissipation associated with a generally higher voltage drop across the device, or high turn-off drive levels which adds further levels of complication and therefore generally makes them even less desirable than the bipolar transistor with its shortcomings.